The galvanization of the exterior surface of pipe or conduit as part of the continuous manufacture thereof from an endless strip of sheet metal has been practiced commercially for a number of years. The process basically consists of roll-forming the metal strip into tubular form after drawing it from an endless supply, welding the seam, scarfing and dressing off the weld, and passing the continuously formed tube through a pickling bath and rinse. The tube is then passed through a preheating station and then through a bath of molten zinc, after which the excess zinc is removed, the tube cooled to handling temperature in a water bath, and the tube sheared into finite lengths.
Such an integrated continuous manufacturing process is disclosed, for example, in U.S. Pat. No. 3,226,817, with particular emphasis on the galvanization step of the process in U.S. Pat. Nos. 3,226,817, 3,259,148 and 3,877,975.
In the galvanizing stations of such prior integrated processes, the continuously-formed, rapidly moving tube, after appropriate preparation, was passed through an elongated trough positioned above a pool of molten zinc in a large vat, from which a stream of the liquid metal was pumped to maintain a substantial and overflowing body of molten zinc in the trough as well as to replace the zinc being carried away from the trough as a fluid coating on the tube.
The amount of zinc pumped from the vat to the upper trough was substantial, and as those skilled in the art will appreciate, the formation of dross at the walls of the vat and the trough, and their consequent erosion due to the scouring action of the recirculating zinc, was likewise substantial. The accelerated erosion of the pump impeller and pump housing in this strenuous service required their replacement in days rather than weeks, but was regarded as a necessary maintenance burden to be tolerated as part of the continuous integrated manufacture of galvanized pipe and tube.